This invention relates to an improved process for the synthesis of certain vinyldioxo compounds.
Vinyldioxo compounds (also referred to herein as VDO compounds).are well known. All are cyclic acetals, extensively studied and patented by Ikeda (U.S. Pat. Nos. 3,010,918; 3,010,923; 3,197,484), by Brachman(U.S. Pat. No. 3,014,924), and by others and described in detail by Hochberg (JOCCA 48, 11, 1043-1068, 1965). The simplest compounds in this class are made by a reaction of acrolein with a compound having two hydroxyl groups, either on adjacent carbon atoms or on carbon atoms separated by an additional carbon atom. When more than two hydroxyl groups are present in a compound, different pairs of hydroxyl groups can react with the aldehyde to form a cyclic acetal. Typical compounds having at least two hydroxyl groups include, for example, ethylene glycol, glycerin, 1,2,6-hexanetriol, and trimethylolpropane. Depending on the number and type of hydroxyl groups, the resulting VDO can be either a substituted 1,3-dioxolane or a substituted 1,3-dioxane, but frequently it is a mixture of a dioxolane with a dioxane. The reaction of acrolein (1) with trimethylolpropane (2) is shown below in Equation 1. The formation of VDO compounds, like other acetal-forming reactions, is catalyzed by acids. ##STR1##
The reaction product of formula (3) is a 2-vinyl-1,3-dioxane substituted in the 1-position with an ethyl group and with a hydroxymethyl (methylol) group.
VDO compounds and their various derivatives have been described as useful polymerizable materials, which have the potential of providing both pigmented and clear-coat finishes in automotive and other applications. Those compounds polymerize in the presence of oxygen, such reactions being catalyzed by cobalt compounds. In an aqueous medium and in the presence of acids, the VDO compounds are unstable, so that further reactions are carried out in either a neutral, an alkaline, or an organic medium. Hochberg (op. cit.) discusses various syntheses and properties of many VDO compounds. In spite of the great industrial potential of VDO compounds, they have not been successfully commercialized in high performance coatings.
Improvements in the general process for making VDO compounds have been described or patented, most recently by Hoepp et al. (U.S. Pat. No. 5,216,179). Most prior art processes give the desired cyclic acetals in yields varying from about 60% to about 90%. For environmental reasons, it is desirable to carry out the synthesis at reasonably high concentrations of starting materials, using moderate amounts of volatile solvents and avoid releasing those solvents into the atmosphere There is, therefore, a need for a process that is environmentally friendly and produces well defined, single VDO compounds in virtually quantitative yields.